netdev - Re: [PATCH net-next v3] virtio_net: add support for Byte Queue Limits

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Message-ID: <ZnJwbKmy923yye0t@nanopsycho.orion>
Date: Wed, 19 Jun 2024 07:45:16 +0200
From: Jiri Pirko <jiri@...nulli.us>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com, virtualization@...ts.linux.dev,
	ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
	john.fastabend@...il.com, dave.taht@...il.com,
	kerneljasonxing@...il.com, hengqi@...ux.alibaba.com
Subject: Re: [PATCH net-next v3] virtio_net: add support for Byte Queue Limits

Tue, Jun 18, 2024 at 08:18:12PM CEST, mst@...hat.com wrote:
>This looks like a sensible way to do this.
>Yet something to improve:
>
>
>On Tue, Jun 18, 2024 at 04:44:56PM +0200, Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@...dia.com>
>> 

[...]


>> +static void __free_old_xmit(struct send_queue *sq, struct netdev_queue *txq,
>> +			    bool in_napi, struct virtnet_sq_free_stats *stats)
>>  {
>>  	unsigned int len;
>>  	void *ptr;
>>  
>>  	while ((ptr = virtqueue_get_buf(sq->vq, &len)) != NULL) {
>> -		++stats->packets;
>> -
>>  		if (!is_xdp_frame(ptr)) {
>> -			struct sk_buff *skb = ptr;
>> +			struct sk_buff *skb = ptr_to_skb(ptr);
>>  
>>  			pr_debug("Sent skb %p\n", skb);
>>  
>> -			stats->bytes += skb->len;
>> +			if (is_orphan_skb(ptr)) {
>> +				stats->packets++;
>> +				stats->bytes += skb->len;
>> +			} else {
>> +				stats->napi_packets++;
>> +				stats->napi_bytes += skb->len;
>> +			}
>>  			napi_consume_skb(skb, in_napi);
>>  		} else {
>>  			struct xdp_frame *frame = ptr_to_xdp(ptr);
>>  
>> +			stats->packets++;
>>  			stats->bytes += xdp_get_frame_len(frame);
>>  			xdp_return_frame(frame);
>>  		}
>>  	}
>> +	netdev_tx_completed_queue(txq, stats->napi_packets, stats->napi_bytes);
>
>Are you sure it's right? You are completing larger and larger
>number of bytes and packets each time.

Not sure I get you. __free_old_xmit() is always called with stats
zeroed. So this is just sum-up of one queue completion run.
I don't see how this could become "larger and larger number" as you
describe.


>
>For example as won't this eventually trigger this inside dql_completed:
>
>        BUG_ON(count > num_queued - dql->num_completed);

Nope, I don't see how we can hit it. Do not complete anything else
in addition to what was started in xmit(). Am I missing something?


>
>?
>
>
>If I am right the perf testing has to be redone with this fixed ...
>
>
>>  }
>>  

[...]